With development of air conditioning technology and enhancement of people's environmental protection awareness, a heat recovery VRF (Variable Refrigerant Flow) air conditioning system becomes increasingly popular in the market. When the heat recovery VRF air conditioning system is in a heating mode, a refrigerant contained in an outdoor unit serving as an evaporator is less than that contained in the outdoor unit serving as a condenser when in a cooling mode, and the redundant refrigerant is usually stored in a gas-liquid separator. Specifically, in the heating mode, the refrigerant is throttled and depressurized by a throttling element in a refrigerant flow direction switching device, and then forms a gas-liquid two-phase refrigerant to enter a pipe; more gaseous refrigerant is stored in the pipe, so that the amount of the refrigerant stored in the pipe is little, while more liquid refrigerant is stored in the gas-liquid separator, and even overflows the gas-liquid separator, which directly results in a drop of a suction-superheat degree of the compressor.
In order to increase the suction-superheat degree of the compressor, in the prior art, an exhaust port of the compressor is usually in communication with a gas return port thereof via a hot gas bypass solenoid valve. When the suction-superheat degree of the compressor decreases, the hot gas bypass solenoid valve is opened to increase the suction-superheat degree, which however directly reduces energy efficiency of the heat recovery VRF air conditioning system.